(a) Field of the Invention
The present invention relates to a wireless access system. More particularly, the present invention relates to a method of performing handover in a wireless access system supporting multi-carrier operation.
(b) Description of the Related Art
At present, research on wireless access systems supporting multi-carriers (MC) is actively under way. A multi-carrier operation allows for more efficient communication compared to a single-carrier operation. For instance, a terminal can use a wider bandwidth, and a base station can serve more users.
In a general multi-carrier operation, two types of carriers, i.e., a primary carrier and a secondary carrier are defined. The primary carrier means a carrier for transmitting and receiving data and a control message between a terminal and a base station. The primary carrier can be used for control and management of the terminal by base station, such as network entry. The secondary carrier means an additional carrier that can be used for data transmission and reception based on the control message transmitted through the primary carrier. The terminal is controlled through the primary carrier, which is one of multi-carriers, and can dynamically use one or more of secondary carriers for data transmission and reception.
The multi-carriers are identified by physical carrier indexes between the base station and the terminal, and can be identified additionally by a logical carrier indexes when they are being used for data transmission and reception. The physical carrier index is the index of a carrier that can be used for the base station.
When a terminal wants to move out of a coverage area of a serving base station with which the terminal is currently in communication and enters a coverage area of another base station, a communication path is changed to a target base station to maintain communication. This procedure is called “handover”. A method of performing handover using multi-carriers has a problem in that a method of performing handover using a single carrier cannot be applied as it is. For instance, the terminal does not know whether the target base station supports multi-carriers and which one of the multi-carriers can be used in order to communicate with the corresponding target base station. Accordingly, there is a need to define a new method of performing handover using multi-carriers.